A neutral-point-clamped (NPC) power converter apparatus is configured such that at least four switching elements are connected in series, and can output a high voltage of several kV˜several-ten kV. In addition, since the neutral-point-clamped power converter apparatus outputs phase voltages of three levels, a higher harmonic wave that occurs is small. Thus, the ripple of the waveform is small, and a current with a smooth waveform is supplied to loads. In addition, of the above-described four switching elements, two elements are switched at the same timing, and the other two are not switched. Thus, the switching loss of the apparatus as a whole is small. Therefore, the neutral-point-clamped power converter apparatuses are widely used in railway cars, driving apparatuses for industrial uses, and power system voltage stabilizing apparatuses.
In the case of using the neutral-point-clamped power converter apparatus having the above-described features, neutral-point potential control is required. The neutral-point potential control is a control to equally divide a DC input voltage by a high-potential-side capacitor and a low-potential-side capacitor, and to equally hold a high-potential-side DC voltage and a low-potential-side DC voltage. In the case where the neutral-point-clamped power converter apparatus is applied to a two-phase output or a three-phase output, there is known a method of executing the neutral-point potential control by applying to the respective phases an identical voltage (zero-phase voltage) which is calculated from voltage instruction values and load currents of the respective phases.
However, when the load current is small, for example, when the load current has a value close to zero, a higher harmonic wave current is relatively greater than a fundamental wave current. Thus, there is a case in which the sign of the fundamental wave current, which contributes to a neutral-point potential variation, and the sign of a detected load current are different. As a result, in some cases, the sign of the calculated zero-phase voltage is reverse to the sign of the voltage that is to be normally applied to the apparatus, and the high-potential-side DC voltage and low-potential-side DC voltage are not balanced.